Authorities say explosives experts are trying to determine what caused an explosion that killed a woman and injured three other people at a Southern California medical office building.

The Tuesday afternoon blast on the first floor of an Aliso Viejo building in Orange County shattered windows and forced children in a nearby daycare center to be evacuated.

Mary McWilliams of San Clemente tells the Orange County Register that she saw two women with burns stagger out of the building. Fire officials say a third person suffered smoke inhalation.

Orange County sheriff's experts along with FBI, the ATF and other federal agencies are investigating the cause of the blast. Orange County Sheriff's Cmdr. Dave Sawyer said Tuesday night that the size of the explosion makes it suspicious.

However, he says no explosive device has been found in the building and investigators still aren't saying whether the explosion was accidental or intentional.

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7:10 p.m.

A witness describes seeing two women with burns following an explosion that killed one person and injured three at a Southern California medical office building.

Mary McWilliams tells the Orange County Register that she arrived for an appointment Tuesday in Aliso Viejo to find smoke surrounding the area and car alarms going off.

McWilliams says she saw two women covered in ash and soot stagger out of the building. McWilliams says one woman had skin peeling on her arm. The other woman was burned and her head was bleeding.

The injured were taken to a hospital and there's no immediate word on their conditions.

Federal, state and local authorities, including the FBI and the Department of Homeland Security, are investigating the blast, which occurred on the first floor of a building that was under renovation.